Plants and factories utilize process control devices to control the flow of fluids in processes, wherein “fluids” may include liquids, gases, or any mixture that can flow through a pipe. Manufacturing processes that create consumer articles or goods such as fuel, food, and clothes require control valves to control and regulate fluid flow. Even a medium sized factory may utilize hundreds of control valves to control a process. Control valves have been utilized for over a century, during, which time valve designers have continuously improved the operational performance of control valves.
When designing a process, the designer is faced with many design requirements and design constraints. For example, some process control applications require a valve to enable flow in two directions, which are often called bi-directional flow valves. Another example of a design constraint includes the pressure at which the fluid will be operating within the process. For example, some processes operate at relatively low pressures, e.g., less than approximately 10,000 pounds per square inch gauge (psig), while other processes may operate at relatively high pressures, e.g., greater than 10,000 psig, and up to approximately 20,000 psig.
FIG. 1 depicts one example of a conventional bi-directional control valve 10. More particularly, the bi-directional control valve 10 of FIG. 1 includes an air-operated bi-directional control valve 10 in that a pneumatic supply is used to control the control valve 10.
The conventional bi-directional control valve 10 generally includes a valve body 12 and an actuator assembly 14. The actuator assembly 14 contains a control element 16 that is adapted for sliding displacement within the valve body 10 between a closed position, which is illustrated in FIG. 1, and an open position (not shown).
More specifically, the valve body 12 includes an inlet 18, an outlet 20, and a throat 27. The throat 22 carries a valve seat 24 for being engaged by the control element 16 to close the valve 10, as depicted. The actuator assembly 14, as mentioned, includes the control element 16. Additionally, the actuator assembly 14 includes a housing 26, a valve insert 28, and a spring 30. The control element 16 generally includes a stem portion 32 and a piston 34. The stem portion 32 extends through the throat 22 of the valve body 12 and includes a reduced diameter portion 32a defining a generally frustoconical seating surface 36. The seating surface 36 engages the valve seat 24 when the control element 16 is in the closed position.
The valve insert 28 is constructed of 316 stainless steel and defines a bore 38 slidably receiving a portion of the stem portion 32 of the control element 16. The bore 38 in the conventional valve insert 28 includes a diameter that is slightly larger than an outer diameter of the stem portion 32 such that the stem portion 32 may freely reciprocate in the bore 38. Additionally, however, the diameter of the bore 38 is substantially equal to a guide bore 48 in the valve body 12 and slightly smaller than the diameter of a bore 25 in the valve seat 24. For example, in the depicted valve 10, the bore 38 in the valve insert 28 and the guide bore 48 in the valve body 12 include diameters of approximately twenty-five hundredths of an inch (0.25″), while the bore 25 in the valve seat 24 includes a diameter of approximately two-hundred and fifty-seven thousandths of an inch (0.257″).
The spring 30 is disposed between the valve insert 28 and the piston 34 such as to bias the piston 34, and therefore the control element 16, upward relative to the orientation of the valve 10 of FIG. 1, and into the closed position. In the conventional valve 10 depicted, the spring may have a load height of approximately sixty-six hundredths of an inch (0.66″) and can generate approximately seventy pounds-force (70 lbf) in the closing direction. The housing 26 is threadably attached to the valve body 12 and maintains the positional relationship of the other components.
As depicted in FIG. 1, the piston 34 of the control element 16 is slidably disposed within a piston cavity 40 defined by the housing 26. The housing 26 additionally defines a threaded aperture 42 for threadably receiving a supply line (not shown) connected to a pneumatic supply, for example, a source of compressed shop-air supplied at a pressure of between approximately eighty (80) psig and approximately one-hundred and fifty (150) psig. The force required to move the piston 34 is a function of the surface area of the piston 34. In the disclosed conventional valve 10, the piston 34 includes a diameter of approximately one and three-hundred and seventy-five thousands of an inch (1.375″). This provides a surface area sufficient for the compressed shop-air to displace the piston 34 as required.
So configured, the position of the control element 16 within the control valve 10 can be controlled by introducing compressed air into the piston cavity 40. For example, in the absence of compressed air supplied to the cavity 40, the spring 30 biases the piston 34 into the position depicted in FIG. 1, which causes the seating surface 36 of the stem portion 32 to sealingly engage the valve seat 24 and close the valve 10. However, the introduction of compressed air into the cavity 40 increases the pressure in the cavity 40 above the piston 34, which then causes the piston 34 and the entire control element 16 to displace downward relative to the orientation of the valve depicted in FIG. 1. Accordingly, the seating surface 36 of the stem portion 32 disengages from the valve seat 24 and opens the valve 10 to allow fluid to flow therethrough.
In some conventional applications, when the valve 10 is closed, as depicted in FIG. 1, the fluid process is such that pressure remains built up within the system. Accordingly, the valve 10 experiences an inlet pressure PI at the inlet 18 of the control valve body 12 and an outlet pressure PO at the outlet 20 of the valve body 12. The inlet pressure PI may be equal to, less than, or greater than the outlet pressure PO at any given instance, or for any given application. The conventional valve 10 depicted is adapted for low pressure applications, where the inlet and outlet pressures PI, PO may rise to approximately 10,000 psig. In high pressure applications, however, the inlet and outlet pressures may rise to between approximately 10,000 psig and approximately 20,000 psig. Therefore, as shown in FIG. 1, the conventional control valve 10 further comprises an upper o-ring 44 and a lower o-ring 46 disposed around the stem portion 32 of the control element 16.
The upper o-ring 44 closes any gap between the stem portion 32 and the bore 38 in the valve insert 28, thereby providing a fluid-tight seal. Thus, the upper o-ring 44 has an outside diameter that is approximately equal to the diameter of the bore 38, i.e., twenty-five hundredths of an inch (0.25″). The lower o-ring 46 closes any gap between the stem portion 32 and the guide bore 48 formed in the valve body 12. Therefore, the bottom o-ring 46 has an outer diameter substantially equal to the diameter of the guide bore 48 in the valve body 12, i.e., twenty-five hundredths of an inch (0.25″). The upper o-ring 44 prevents the fluid at the outlet 20 of the valve body 12, which is compressed at the outlet pressure PO, from leaking between the stem portion 32 and the valve insert 28. The lower o-ring 46 prevents the fluid at the inlet 18 of the valve body 12, which is compressed at the inlet pressure PI, from leaking between the stem portion 32 and the valve body 12 and into the guide bore 48.
As mentioned above, in the absence of compressed air being supplied to the piston cavity 40 of the housing 26, the spring 30 biases the control element 16 into the closed position, as depicted in FIG. 1, such that the seating surface 36 seats against the valve seat 24. The spring 30 therefore helps establish the character and integrity of this seal.
Additionally, the inlet pressure PI may help the character and integrity of this seal. For example as shown in FIG. 1, the inlet pressure PI acts on the portion of the stem portion 32 that is disposed below the valve seat 24. Specifically, the stem portion 32 further includes a shoulder 50 disposed just below the seating surface 36. The shoulder 50 defines a ring-shaped surface 51 that is disposed substantially perpendicular to a longitudinal axis of the stem portion 32. The inlet pressure PI acts on this shoulder 50 and applies an upward force to the valve stem portion 32 relative to the orientation of the valve 10 in FIG. 1. This force therefore helps seat the seating surface 36 of the stem portion 32 against the valve seat 24. The inlet pressure PI also acts on the lower o-ring 46 to apply a force to the stem portion 32 in an opposite direction, i.e., the downward direction relative to the orientation of the valve 10 in FIG. 1. However, because an area of the shoulder 50 that is acted on by the inlet pressure PI is larger than an area of the lower o-ring 46 that is acted on by the inlet pressure PI, a sum of the forces generated by the inlet pressure PI on the stem portion 32 below the valve seat 24 results in a net force applied upward on the stem 32.
By contrast, the outlet pressure PO at the outlet 20 of the valve body 12 acts on the portion of the stem portion 32 above the valve seat 24. More specifically, as mentioned above, the stem portion 32 includes the reduced diameter portion 32a. Additionally, the stem portion 32 defines a second frustoconical surface 52 disposed opposite the reduced diameter portion 32a from the seating surface 36. Therefore, the outlet pressure PO acts on a portion of the seating surface 36 that is located within the bore 25 of the valve seat 24 to apply a force to the stem portion 32 in a downward direction relative to the orientation of the valve 10. The outlet pressure PO also acts on the second frustoconical surface 52 to apply a force to the stem in the upward direction relative to the orientation of the valve 10. Moreover, the outlet pressure PO acts on a portion of the upper o-ring 44 carried by the stem portion 32 between the stem portion 32 and the bore 38 in the valve insert 28. The pressure acting on the upper o-ring 44 also generates a force on the stem portion 32 in the upward direction.
However, because the diameter of the bore 38 in the valve insert 28 is smaller than the diameter of the bore 25 in the valve seat 24, the combined area of the second frustoconical surface 52 and the upper o-ring 44 that is acted on by the outlet pressure PO, is less than the area of the portion of the seating surface 36 disposed within the valve seat 24 that is acted on by the outlet pressure PO. Thus, the outlet pressure PO applies a greater force to the stem portion 32 in the downward direction relative to the orientation of the valve 10 in FIG. 1. Therefore, when the outlet pressure PO and the inlet pressure PI are substantially equal, the downward force generated by the outlet pressure PO at least partially negates the upward force generated by the inlet pressure PI. Accordingly, the spring 30 is the sole component serving to ensure that the seating surface 36 of the stem portion 32 remains seated against the valve seat 24.
Typically, the spring 30 is sufficient to provide this function. However, under high pressure conditions, i.e., between approximately 10,000 psig and approximately 20,000 psig, the difference in the forces generated by the inlet and outlet pressures PI, PO and applied to the stem 32, and the effects created thereby, can become substantial. This can compromise the integrity of the seat between the seating surface 36 of the stem portion 32 and the valve seat 24, and therefore, the performance of the valve 10.